1. Field of the Invention
This invention relates to electrical resonators employing a mechanical transducer and more particularly to a method for fine tuning such resonators following batch fabrication.
2. Description of the Related Art
The need to reduce the cost and size of electronic equipment has led to a continuing need for ever smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filter element that meets these needs is constructed from mechanical resonators such as acoustic resonators. These devices use acoustic waves, bulk longitudinal waves for example, in thin film material, typically but not exclusively piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The resonator may be suspended in air, supported along its rim, or may be placed on an acoustic mirror comprised of a plurality of alternating layers of high and low acoustic impedance (the product of speed and density), usually silicon dioxide and aluminum nitride. When an electric field is applied between the two electrodes via an impressed voltage, the PZ material converts some of the electrical energy into mechanical energy in the form of sound waves. For certain crystal orientations, such as having the c axis parallel to the thickness of an Aluminum Nitride film, the sound waves propagate in the same direction as the electric field and reflect off of the electrode/air or electrode/mirror interface.
At a certain frequency which is a function of the resonator thickness the forward and returning waves add constructively to produce a mechanical resonance and because of the coupling between mechanical strain and charge produced at the surface of a piezoelectric material, the device behaves as an electronic resonator; hence, such devices combined in known architectures can act as a filter. The fundamental mechanical resonant frequency is that for which the half wavelength of the sound waves propagating in the device is equal to the total thickness of the piezoelectric plus electrode layers. Since the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be more compact than dielectric cavity resonators. Resonators for 50 Ohm matched applications in the GHz range may be constructed with physical dimensions approximately 100 micrometers in diameter and few micrometers in thickness.
The resonant frequency of the resonator is a function of the acoustic path of the resonator. The acoustic path is determined by the distances between the outer surfaces of the electrodes. When batch producing resonators on a substrate, the thickness of the transducing material and the electrodes is fixed at fabrication; hence, the resultant resonance frequency is also fixed. Since there are variations in thickness from device to device resulting from manufacturing tolerances, some method for fine tuning the resonance frequency of each device is needed.
To compensate for this inability to reliably and inexpensively mass produce resonators with the proper resonance characteristics, it is known to intentionally produce resonators having a lesser thickness than the thickness indicated to achieve a desirable resonant frequency, and then deposit excess material on at least one of the electrodes to change the overall thickness of the device and thereby fine tune the device. As this deposition of material may be done while the device is subjected to an input signal and simultaneously tested for resonance this method has produced acceptable results.
This method is not, however without problems as the presence of a mask needed to control the deposition over the desired electrodes creates problems of its own. If the mask, for instance is in contact with the electrode, the mask mass is added to the device mass and alters the resonance characteristics of the device. On the other hand if the mask is not in contact with the device the control of the deposition area suffers. Such masking techniques have been successful with quartz type resonators that are much larger, but have not been as successful with resonators of the order of less than one millimeter.
It has also been proposed to remove material from the device in order to adjust its resonant frequency by etching material off the top electrode of a resonator. With current technology, however, etching is not as controlled a process as deposition. Etching tends to be less uniform, smooth or reproducible than deposition. In fact prolonged etching may in cases change the composition, morphology, grain nature or roughness of thin films. Accurate etching processes require precise knowledge of the rate at which material is removed to permit stopping at the exact moment that sufficient material has been removed to produce the desired resonant frequency. To a certain extent lack of precise control of the etching rate may be alleviated by monitoring the device frequency during the etching process.
When removal of material is done in a dry etching process it is usually possible to monitor the resonant frequency of the device during the etching process. However, monitoring of the resonant frequency during etching is not possible when wet etching processes are used. Wet processes are desirable as they are much faster than dry processes.
There is thus still a need for a process to accurately fine tune a mechanical resonator to a desired frequency without concern for possible over-etching and without need to monitor the frequency during the etching process.